Refrigeration & Air-Conditioning Practice Set-3

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1. The atmospheric air at dry bulb temperature of 15°C enters a heating coil maintained at 40°C. The air leaves the heating coil at 25°C. The bypass factor of the heating coil is

0.376

0.6

0.4

0.67

2. The refrigerant after condensation process is cooled below the saturation temperature before throttling. Such a process is called

Sub-cooling or under-cooling

Normal cooling

Super-cooling

None of these

3. Freon group of refrigerants are

Toxic

Nontoxic and non-inflammable

Non-inflammable and toxic

Inflammable

4. Which of the following cycles uses air as the refrigerant?

Carnot

Bell Coleman

Ericson

Stirling

5. The minimum temperature to which water can be cooled in a cooling tower is

Dew point temperature of air

Wet bulb temperature of air

Ambient air temperature

Dry bulb temperature of air

6. The curved lines on a psychrometric chart indicates

Dry bulb temperature

Dew point temperature

Relative humidity

Wet bulb temperature

7. The C.O.P. of a refrigerator working on a reversed Carnot cycle is (where T₁ = Lowest absolute temperature, and T₂ = Highest absolute temperature)

T₁ / (T₂ - T₁)

(T₂ - T₁)/T₁

(T₁ - T₂)/T₁

T₂/ (T₂ - T₁)

8. In a flooded evaporator refrigerator, an accumulator at suction of compressor is used to

Collect liquid refrigerant and prevent it from going to compressor

Detect liquid in vapour

Superheat the vapour

Collect vapours

9. The operating pressure for refrigerating units using R-12 as a refrigerant is

8 bar

15 bar

30 bar

2 bar

10. Critical temperature is the temperature above which

Water will never evaporate

A gas will never liquefy

A gas will immediately liquefy

Water will evaporate

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11. The change in evaporator temperature in a refrigeration cycle, as compared to change in condenser temperature, influences the value of C.O.P.

Unpredictable

Less

More

Equally

12. The condition of refrigerant after passing through the condenser in a vapour compression system is

Superheated vapour

Dry saturated vapour

Saturated liquid

Wet vapour

13. A bootstrap air cooling system has

Three heat exchangers

Four heat exchangers

One heat exchanger

Two heat exchangers

14. The value of C.O.P in vapour compression cycle is usually

Any one of the above

Equal to unity

Always less than unity

Always more than unity

15. In a refrigeration system, the expansion device is connected between the

Evaporator and compressor

Receiver and evaporator

Compressor and condenser

Condenser and receiver

16. In actual air-conditioning applications for R-12 and R-22, and operating at a condenser temperature of 40° C and an evaporator temperature of 5° C, the heat rejection factor is about

5.12

2.15

1.25

1

17. The vapour compression refrigerator employs the following cycle

Rankine

Carnot

Reversed Carnot

Reversed Rankine

18. The freezing point of sulphur dioxide is

-56.6°C

-135.8°C

-77.7°C

-75.2°C

19. The domestic refrigerator uses following type of compressor

Centrifugal

Miniature sealed unit

Piston type reciprocating

Axial

20. On the pressure-enthalpy diagram, condensation and desuperheating is represented by a horizontal line because the process

Takes place at constant pressure

Involves no change in volume

Takes place at constant entropy

Takes place at constant temperature

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21. Mass flow ratio of NH₃ in comparison to Freon-12 for same refrigeration load and same temperature limits is of the order of

1: 9

1: 1

1: 3

9: 1

22. The bank of tubes at the back of domestic refrigerator is

Condenser tubes

Evaporator tubes

Capillary tubes

Refrigerant cooling tubes

23. Which of the following is not a desirable property of a refrigerant?

Low boiling point

Good electrical conductor

High risibility with oil

Large latent heat

24. Which of the following statement is correct?

(D) Both (A) and (B)

(C) The warm air increases the rate of radiation of heat from the human body.

(B) The increase in air movement increases the evaporation from the human body.

(A) The human body can lose heat even if its temperature is less than the atmospheric temperature.

25. In a refrigeration system, heat absorbed in comparison to heat rejected is

More for small capacity and less for high capacity

More

Less

Same

26. A certain refrigerating system has a normal operating suction pressure of 10 kg/cm gauge and condensing pressure of about 67 kg/cm. The refrigerant used is

Ammonia

Freon

Carbon dioxide

Brine

27. When the temperature of the surrounding is higher than the temperature of the body, then the heat loss by convection from the body to the surrounding will be

Negative

None of these

Zero

Positive

28. The boiling point of ammonia is

-33.3°C

-77.7°C

-30°C

-10.5°C

29. The capillary tube, as an expansion device, is used in

All of these

Domestic refrigerators

Room air conditioners

Water coolers

30. If a heat pump cycle operates between the condenser temperature of +27°C and evaporator temperature of -23°C, then the Carnot COP will be

0.2

6

5

1.2

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31. The general rule for rating refrigeration systems (excepting for CO₂ system) is to approximate following h.p. per ton of refrigeration

0.1 to 0.5 h.p. per ton of refrigeration

0.5 to 0.8 h.p. per ton of refrigeration

2 to 5 h.p. per ton of refrigeration

1 to 2 h.p. per ton of refrigeration

32. Cooling water is required for following equipment in ammonia absorption plant

Absorber

Evaporator

Condenser

Condenser, absorber and separator (rectifier)

33. For obtaining high COP, the pressure range of compressor should be

High

Any value

Low

Optimum

34. A thermostatic expansion valve in a refrigeration system

Ensures the evaporator completely filled with refrigerant of the load

Is suitable only for constant load systems

None of the above

Maintains different temperatures in evaporator in proportion to load

35. A human body feels comfortable when the heat produced by the metabolism of human body is equal to the

(B) Heat stored in the human body

(A) Heat dissipated to the surroundings

(D) Difference of (A) and (B)

(C) Sum of (A) and (B)

36. A reversible engine has ideal thermal efficiency of 30%. When it is used as a refrigerating machine with all other conditions unchanged, the coefficient of performance will be

3.33

1.33

4.33

2.33

37. Rating of a domestic refrigerator is of the order of

0.1 ton

5 tons

40 tons

10 tons

38. In a lithium bromide absorption refrigeration system

None of these

Ammonia is used as a refrigerant and lithium bromide as an absorbent

Water is used as a refrigerant and lithium bromide as an absorbent

Lithium bromide is used as a refrigerant and water as an absorbent

39. Highest temperature encountered in refrigeration cycle should be

Near critical temperature of refrigerant

Above critical temperature

At critical Temperature

Much below critical temperature

40. Most thermostatic expansion valves are set for a superheat of

20°C

5°C

15°C

10°C

Refrigeration & Air-Conditioning Practice Set-3

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